Cyclophosphazenes substituted with allylphenoxy are known to be usable in flame retardants, flame-retardant resin compositions, and also molded articles, electronic components, etc., using these resin compositions (see Patent Literature 1 and 2). Cyclophosphazenes show promise for various applications, in particular, because the double bond of their allyl group undergoes Diels-Alder cycloaddition reaction with a dienophile such as bismaleimide, thereby providing excellent thermosetting polymers.
Chlorocyclophosphazenes, a starting material for producing cyclophosphazenes, are produced typically by reacting phosphorus pentachloride with ammonium chloride (or ammonia gas) in an organic solvent, and the product obtained by this method is a mixture of chlorocyclophosphazenes in the form of trimer to pentadecamer.
Cyclophosphazenes substituted with allylphenoxy have been produced using chlorocyclophosphazenes (a starting material) of a uniform degree of polymerization, which are obtained by purifying the mixture of chlorocyclophosphazenes by a technique such as distillation or recrystallization.
However, the yield of the trimer, which is the predominant form, in this production method is merely 50% or less, based on the phosphorus pentachloride, and non-trimeric chlorocyclophosphazenes that could not be separated by purification were inevitably wasted without being used.
Given the current status of the art, it is considered economically preferable to produce cyclophosphazenes substituted with allylphenoxy from a mixture of chlorocyclophosphazenes without the need for isolating chlorocyclophosphazenes of a desired degree of polymerization and to use the product (mixture) for various purposes.
The present inventors examined commonly used methods for introducing allylphenoxy into a chlorocyclophosphazene, such as the method disclosed in Patent Literature 1 or other similar methods. Typically, to produce a strong thermosetting polymer, at least two allylphenoxy groups are preferably present in one molecule, but the examination found that none of the trimers or tetramers in the chlorocyclophosphazene mixture was substituted with allylphenoxy, or that a compound substituted with only one allylphenoxy group per molecule was merely obtained. The probable reason for this is that chlorocyclophosphazenes of a higher degree of polymerization exhibit a higher reactivity, and an added allylphenolate compound first reacts with such highly polymerized chlorocyclophosphazenes and is thus consumed, leaving the low-polymerized trimers and tetramers unsubstituted with allylphenoxy, and that these trimers and tetramers then proceed to the subsequent reaction with an unsubstituted phenolate compound or a phenolate compound substituted with unreactive groups.
The inventors also attempted to react a mixture of chlorocyclophosphazenes in accordance with the method disclosed in a reference document (see Non-patent Literature 1), in which an unsubstituted phenolate compound is first allowed to act on chlorocyclotriphosphazenes, and subsequently an allylphenolate compound is allowed to act on the resultant. Unlike the results of the method disclosed in the reference document, hexaphenoxy-cyclophosphazenes that were fully substituted at their replaceable positions were generated because the unsubstituted phenolate compound with a smaller steric hindrance than allylphenolate compounds was less selective.